High-strength steel sheets are used for bodies of automobiles, and techniques relating to these kinds of steel sheets are mentioned below. Japanese Unexamined Patent Application Publication No. 2002-97545 discloses a steel sheet with high-workability and high-strength having superior shape-retaining properties in machining processing and absorption properties for impact energy. A steel sheet of a specified composition has a complex structure including a residual austenite which is not less than 3% by volume, an average ratio of X-ray random reinforcement of the orientation group {1 0 0}<0 1 1> to {2 2 3}<1 0 0> on at least an area at a depth of ½ sheet thickness from the surface is not less than 3.0, an average ratio of X-ray random reinforcement of three crystal orientations {5 5 4}<2 2 5>, {1 1 1}<1 1 2> and {1 1 1}<1 1 0> is not more than 3.5, and at least one plastic strain ratio in the directions which are a rolling direction and a direction perpendicular to the rolling direction is not more than 0.7.
Japanese Unexamined Patent Application Publication No. 10-147838 discloses a high-strength steel sheet consisting of 0.05 to 0.20 wt % of C, 2.0 wt % or less of Si, 0.3 to 3.0 wt % of Mn, 0.1 wt % or less of P, 0.1 wt % or less of Al, and the balance of Fe and inevitable impurities. The steel sheet has two phase structures of a martensitic phase and the balance of a ferrite phase. Volume rate of the martensitic phase is 5 to 30%, and a ratio Hv (M)/Hv (F) in which Hv (M) is hardness of martensitic phase and Hv (F) is hardness of ferrite phase, is 3.0 to 4.5.
Japanese Unexamined Patent Application Publication No. 2000-73152 discloses a production method for high-strength metal sheets comprising an ultrafine structure that is refined to an average grain size of not more than 1 μm by repeating plural cycles of the following processes. The processes includes a step for laminating plural metal sheets, of which the surface is cleaned, and connecting the edges thereof, a step for heating the laminated sheets having connected edges in a range of a recovery temperature and below a recrystallizing temperature, a step for rolling and connecting the heated laminated sheets into a predetermined sheet thickness, and a step for cutting the laminated sheets which are connected by rolling into a predetermined length in a longitudinal direction, thereby making plural metal sheets, and cleaning surfaces thereof.
Japanese Unexamined Patent Application Publication No. 2002-285278 discloses a low-carbon steel with high-strength and high-ductility having properties in which the tensile strength is not less than 800 MPa, the average elongation is not less than 5%, and the elongation is not less than 20%. Such a steel may be obtained by the following processes. A plain low-carbon steel or a plain low-carbon steel with not more than 0.01% of boron in a range which is an effective amount for accelerating martensitic transformation is processed and heated. Then, the steel having not less than 90% of a martensitic phase, which is obtained by water-cooling after coarsening the austenite grains, is worked under low strain. Specifically, the steel is subjected to cold rolling at an overall reduction rate of 20% or more, but less than 80%, and low-temperature annealing at a temperature of 500 to 600° C., thereby obtaining an average grain size of a ferrite structure of ultrafine grains which is not more than 1.0 μm.
Generally, increasing the strength of the steel sheet for automobile bodies and improving the absorption characteristics of impact energy are effective to protecting occupants from the impact of automobile crashes. However, when the strength of the steel sheet is simply increased, the workability decreases and the press forming is difficult to perform. Therefore, both the press formability and the impact energy absorption properties are generally improved by increasing the difference of static and dynamic stresses which are generated in the static deformation corresponding to the press forming and are generated in the dynamic deformation corresponding to the impact.
That is, the above Japanese Unexamined Patent Application Publication No. 2002-97545 proposes a steel sheet comprising a complex structure of ferrite and residual austenite as a steel sheet with a large difference of static and dynamic stresses. According to the technique shown in the above reference (p. 13, Table 2), for example, a steel sheet in which the stress of the static deformation is 784 MPa and the difference of static and dynamic stresses is 127 MPa may be obtained. However, the difference of static and dynamic stresses are lower than that of mild steel sheets. Conventionally, a high-strength steel sheet in which stress of the static deformation exceeds 500 MPa was impossible to have difference of static and dynamic stresses of not less than 170 MPa, which corresponds to that of mild steel sheets.
The reason for this is explained below. A large number of alloying elements needed to be added to a mild steel sheet as a raw material, in order to increase the strength by conventional methods, that is, solid solution strengthening, precipitation strengthening, complex structure strengthening, and quench strengthening. Therefore, the purity of the ferrite is low when the series of the methods are applied. The difference of static and dynamic stresses of the ferrite depends on a thermal component generated by thermal oscillation of atoms, which is a portion of the potential amount required for movement of dislocation. The dependence of the strain rate of the deformation stress increases when the thermal component is large. However, the dependence of the strain rate of the deformation stress decreases when the thermal component is small due to the low purity of the ferrite. Therefore, the decrease of the difference of static and dynamic stresses was inevitable when the steel was strengthened by the conventional methods.
In the above Japanese Unexamined Patent Application Publication No. 10-147838, a steel with a complex structure of ferrite and martensite may be strengthened by controlling the amount of solid-solved carbon, which process corresponds to baking painting (2% of pre-strain and heat treatment at 170° C. for 20 minutes). However, the strength is difficult to improve when draw forming is changed to bending forming to simplify the press processes, because the strengths of portions that are not strained are not changed by the method. Moreover, in recent years, baking painting has been performed at lower temperatures and for shorter times, and the above expected effect is difficult to obtain. Therefore, development of steel sheets that have excellent impact energy absorption properties without baking painting has been required.
Under these circumstances, a refinement of ferrite grains is focused on as a method for strengthening steels, which is independent of the above conventional methods. That is, the method is used for strengthening the steel by controlling the addition of alloying elements as little as possible, not by adding alloying elements, but by enlarging the area of grain boundaries, and refining the grains maintaining the high purity of ferrite. The outline of function of the method is that the strain rate of the deformation stress is independent of the grain size, which is measured on the basis that a migratory distance required for one shift of a Peierls potential is independent of the grain size.
The relationship between the grain size and the strength is known from the Hall-Petch equation, and the strength against deformation is proportional to −½ the power of the grain size. According to the equation, the strength is considerably increased when the grain size is less than 1 μm, for example, the strength of the steel when the grain sizes are 1 μm is at least 3 times higher than that of the steel when the grain sizes are 10 μm.
The above Japanese Unexamined Patent Application Publication No. 2000-73152 may be mentioned as an example of a method of refining the grain sizes of ferrite on the order of nanometers, which is smaller than 1 μm, in regard to the steel sheets that can be press formed. In this method, when laminating and rolling is repeated for 7 cycles, the structure becomes an ultrafine structure in which grain sizes are on the order of nanometers and the tensile strength reaches 3.1 times (870 MPa) as high as that of the IF steel which is used as a raw material. However, the method has two drawbacks.
The first drawback is that the ductility of the material is extremely low in the conditions under which the structure is made from only ultrafine grains, of which grain sizes are not more than 1 μm (hereinafter called “nanograins”). The reason for this is mentioned in the paper written by the inventors of the above reference, for example, “Iron and Steel” (The Iron and Steel Institute of Japan, Vol. 88 (2002), No. 7, p. 365, FIG. 6b). That is, the overall elongation greatly decreases, and the average elongation simultaneously decreases to approximately 0, when the grain sizes of ferrite are less than 1.2 μm. Such a structure is not suitable for steel sheets to be press formed.
The second drawback is that the production efficiency is decreased and the production cost thereby increases to a large extent when laminating and rolling is repeated in an industrial process. Large strain is required for the steel sheet in order to have ultrafine grains, and for example, the ultrafine grains are not obtained until 97% of the strain which is in terms of rolling rate is applied by 5 cycles of the laminating and the rolling. The ultra-refinement cannot be practically performed in ordinary cold rolling because the thickness of the steel sheet needs to be rolled from 32 mm to 1 mm thick, for example.